Disc drives enable users of modern computer systems to store and retrieve vast amounts of data in a fast and efficient manner. A typical disc drive houses a number of circular, magnetic discs (such as five to ten) which are axially aligned and rotated by a spindle motor at a constant, high speed (such as 10,000 revolutions per minute). As the discs are rotated, an actuator assembly moves an array of read/write heads out over the surfaces of the discs to store and retrieve the data from tracks defined on the surfaces of the discs.
A closed loop digital servo system is typically used to control the position of the heads relative to the tracks. The servo system generates a position error signal (PES) indicative of the position of the heads from servo information that is written to the discs during the manufacturing of the disc drive. In response to the detected position, the servo system outputs current to an actuator motor (such as a voice coil motor, or VCM) utilized to pivot the actuator assembly, and hence the heads, across the disc surfaces.
It is a continuing trend in the disc drive industry to provide successive generations of disc drive products with ever increasing data storage capacities and data transfer rates. Because the amount of disc surface area available for the recording of data remains substantially constant (or even decreases as disc drive form factors become smaller), substantial advancements in areal recording densities, both in terms of the number of bits that can be recorded on each track as well as the number of tracks on each disc, are continually being made in order to facilitate such increases in data capacity.
The servo information used to define the tracks is written during disc drive manufacturing using a highly precise servo track writer. While the tracks are intended to be concentric, uncontrolled factors such as bearing tolerances, spindle resonances, misalignments of the discs and the like tend to introduce errors in the location of the servo information. Each track is thus typically not perfectly concentric, but rather exhibits certain random, repeatable variations which are sometimes referred to as repeatable runout, or RRO, with the RRO appearing as a error component of the PES.
While RRO has previously had a minimal impact upon the operation of the servo system, RRO has an increasingly adverse affect as higher track densities are achieved. Particularly, RRO can ultimately lead to an upper limit on achievable track densities, as RRO cuts into the available track misalignment budget and reduces the range over which the servo system can provide stable servo control.
Methodologies to eliminate primary sinisoidal RRO components have been disclosed, such as by U.S. Pat. No. 5,402,280 issued Mar. 28, 1995 to Supino and U.S. Pat. No. 5,404,253 issued Apr. 4, 1995 to Painter. These and other methodologies typically comprise the use of a sine table to generate a sinusoidal correction signal of a selected amplitude and frequency which is added to the PES in order to eliminate the primary sinusoidal RRO components therefrom.
However, these and other prior art approaches are insufficient to compensate for largely random, non-sinusoidal (or complex sinusoidal) RRO within the PES. There is a need, therefore, for an improved approach to compensating for random variations in the location of servo information on the discs of a disc drive.